Article pubs.acs.org/JPCC
Influence of Solvent Addition on the Properties of Ionic Liquids M. L. Phung Le,†,‡,∥ Laure Cointeaux,† Pierre Strobel,‡ Jean-Claude Leprêtre,† Patrick Judeinstein,§,⊥ and Fannie Alloin*,† †
LEPMI-UMR 5279 CNRS, Grenoble INP, Université Joseph Fourier, Université de Savoie, BP75, 38402 Saint Martin d’Hères, France ‡ Institut Néel, 38054, Grenoble, France § ICMMO, UMR 8182, Bâtiment 410, Université Paris-Sud, 91405 Orsay Cedex, France ABSTRACT: The effects of the incorporation of ethylene carbonate (EC) or dimethyl carbonate on the physicochemical and electrochemical properties of ionic liquids (ILs) based on aliphatic quaternary ammonium and imide anion were studied. The evolution of the melting point, glass transition, ionic conductivity, diffusion coefficient, and electrochemical stability were evaluated. The addition of a low amount of solvent, that is, 20 wt %, allows us to improve significantly the conductivity values, reaching 12 mS/cm at 40 °C. The incorporation of a polar solvent, EC, has no positive effect on the IL dissociation. Moreover, the incorporation of EC in ILs improves the electrochemical stability toward reduction, whereas the high anodic stability is maintained. The addition of LiTFSI in IL + solvent electrolytes has been investigated. Although this addition reduces the ionic conductivity, this decrease is less pronounced than in pure ILs, showing the beneficial effect of the additive solvent. (SEI)13,14 and transport properties thanks to a decrease of the viscosity.15 In addition, NMR investigations16,17 indicate a large contribution of ionic association in pure ILs. The addition of polar solvents in ILs should develop IL/solvent interactions at the expense of IL−IL ones, contributing to the increase of the ion dissociation. Tokuda et al.18 have shown that the addition of both 1,2-dichloroethane and propylene carbonate improves the ionic association of the N,N-diethyl-N-2-methoxyethyl-Nmethyl ammonium ion with anions. However, the comparison of the molar conductivity of the IL/solvent mixtures and pure ILs at the same viscosity show,19 as expected, the more polar the solvent is, greater is the salt dissociation. To further explore the potential applications of quaternary ammonium (QA)-based ILs, we present, in this paper, the evolution of physicochemical properties of such ILs in the presence of organic solvents. Herein, fundamental characterization is reported, including thermal properties, conductivity, self-diffusion coefficient, and electrochemical stability. The impact of the addition of lithium salt on both conductivity and ionic dissociation is evaluated.
1. INTRODUCTION Room-temperature ionic liquids (ILs) have attracted a strong interest because of their potential as safe solvents for Li-ion batteries and dye-sensitized solar cells.1−3 They display interesting properties, such as nonvolatility, nonflammability, high thermal stability, wide liquid range, and a wide electrochemical window. To obtain low-melting and low-viscous ILs, the main anions used are the bis(trifluoromethanesulfonyl)imide (TFSI−) and4−6 fluorosulfonyl(pentafluoroethanesulfonyl)imide.7−9 Indeed, these fluorinated anions present chemical and electrochemical stabilities, low symmetry, high flexibility, and a weakly coordinating nature. However, even with the imidazolium cation and imide anion, ILs have a high viscosity, which is detrimental for transport properties. One solution to designing safer electrolytes based on ionic liquids that exhibit simultaneously all the required properties (i.e., low viscosity, electrochemical stability, nonflammability) is the addition of molecular additives. The nonflammability of an electrolyte is maintained with the incorporation of organic solvents up to a concentration of 20 wt % in ILs.10 Abdallah et al. have shown that the incorporation of 20 mol % N-trimethyl-N-propylammonium bis(trifluoromethylsulfonyl) imide permits the inflammability of acetonitrile.11 Guerfi et al.12 observed no flammability with the addition of 40% of ethylmethylimidazolium TFSI in an ethylene carbonate−diethylcarbonate (EC− DEC) mixture. Moreover, solvent addition permits one to improve the formation of a suitable solid electrolyte interface © 2012 American Chemical Society
2. EXPERIMENTAL SECTION 2.1. Synthesis. Alkyl ammonium bis(trifluoromethane sulfonyl) imide ionic liquids, ILs, were synthesized by Received: February 9, 2012 Revised: March 13, 2012 Published: March 14, 2012 7712
dx.doi.org/10.1021/jp301322x | J. Phys. Chem. C 2012, 116, 7712−7718
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Scheme 1
1
metathesis reaction of a freshly prepared halide salt of the ammonium with different alkyl chains, as previously reported.17 The water content of all the ILs, determined by Karl Fischer titration, is below 20 ppm. The ILs are stored in a glovebox ([H2O] < 3 ppm). The ethylene carbonate (EC) and the dimethylcarbonate (DMC) (Aldrich) were stored in molecular sieves in a glovebox. 2.2. Measurements. Thermal Measurements. DSC tests were performed using a TA Instruments DSC 2920 CE. Samples of 3 mg were sealed in aluminum pans in a glovebox. Each sample was quenched at −120 °C to avoid crystallization during the cooling step and was heated from −120 to 100 °C at a heating rate of 5 °C·min−1. The melting and crystallization temperatures, Tm and Tc, were taken at the top of the melting and crystallization peaks, respectively. The glass transition temperature, Tg, was taken at the midpoint of the glass transition. Viscosity. The viscosity of the ILs was measured on the ARG2 rheometer from TA Instruments. The ARG2 is a stresscontrolled rheometer, which was equipped with a Peltier temperature-controlled plate and specific chamber to perform experiments under nitrogen in the temperature range of −5 to 60 °C. The viscosity of the samples was measured using 40 mm diameter cylindrical plates, with a gap of approximately 1000 μm. Electrochemical Investigations. Conductivities were determined by electrochemical impedance spectroscopy using an HP 4192A Impedance Analyzer in the frequency range of 5 Hz to 13 MHz. The samples were placed in a dip-type glass cell with two Pt electrodes fixed at a constant distance, and measurements were performed from −10 to 70 °C under argon. The temperature was equilibrated for 1 h before each measurement. The cell constant was determined by using a 0.1 M KCl solution. Cyclic voltammetry (CV) was obtained at ambient temperature in a glovebox. The counter electrode was a Pt wire, and the working electrode was a Pt microelectrode with a diameter of 125 μm. The scanning rate used was 5 mV/s. The reference electrode was a Ag wire in 10 mM AgNO3 in acetonitrile + 0.1 M tetrabutylammonium. The diffusion of the reference solution was prevented through the use of an additional compartment containing the IL. The potential can be converted to the Li/Li+ scale by adding 3.548 V.20 NMR Investigations. NMR measurements were carried out on a 9.4 T Bruker Avance 400 NMR spectrometer equipped with a Bruker 5 mm dual broadband/{1H−19F} probe with a z axis and a temperature controller (stability and accuracy of 0.2 °C). NMR resonance frequencies are 400.13, 376.50, and 155.51 MHz, respectively, for 1H, 19F, and 7Li nuclei. The 3 mm inner diameter tubes were filled with the samples in the glovebox, and the tubes were sealed in order to avoid any contact with the air moisture. The reduction of the tube diameter avoids convection effects. This point was checked by
H PFG-NMR on one of the sample in changing the diffusion delay Δ from 50 to 200 ms. The self-diffusion measurements were performed with the pulsed field gradient stimulated echo and LED sequence using 2 spoil gradients (PFG NMR).21 The magnitude of the pulsed field gradient was varied between 0 and 40 G cm−1, the diffusion time Δ between two pulses was fixed at 100 ms, and the gradient pulse duration δ was set between 3 and 18 ms depending on the diffusion coefficient of mobile species. This allowed us to observe the attenuation of spin echo amplitude over a range of at least 2 decades, leading to a good accuracy ( 2.5 M, (ii) the formation of [Li(TFSI)n+1]n− complexes, leading to an increase of the overall size of the lithium species, and (iii) the solvation of Li+ by EC. In IL + 20 wt % EC + 0.75 M LiTFSI, the molar conductivity of the electrolytes, ΛNMR, is calculated from the self-diffusion coefficients using the expression ΛNMR =
the methyl moieties that are linked to nitrogen of the ammonium. It was explained as a signature of some nanoscale organization inside these electrolytes with segregation of polar and apolar domains.17 The second one shows that Li+ cations are also solvated by EC. However, more quantitative analysis requires the measurement of the whole build-up magnetization curves for the different sites;37 these data are presented in the inset of Figure 4. They show that the two correlations have similar intensities, which evidence that Li+ cations are solvated by EC but also that ionic clusters, [Li(TFSI)n+1]n− remain even when EC is added to the electrolyte. Attempts to measure 7Li−19F failed; if ion pairs are formed, the Li+ cation should lie close to the delocalized negatively charged moiety [SO2NSO2]− and then the distance with the CF3 units would be too large to give a measurable NOE effect.17 Homonuclear 1H−1H NOESY experiments were also recorded; no cross signals were measured, which displayed that EC/ammonium interactions are weak, as depicted by other techniques.31 3.5. Electrochemical Stability. Electrochemical windows of the electrolytes IL + solvent were determined by cyclic voltammetry on a platinum microelectrode. The evolution of the voltammograms obtained in the anodic region with the addition of solvent is given in Figure 5. The study is performed
NAe 2(x LiD Li+ + x ND N+ + DTFSI−) kT
with xLi and xN the molar ratio of LiTFSI and IL, respectively. The results are given in Table 4. The Λimp/ΛNMR values for the ILs + 20 wt % EC + LiTFSI 0.75 M are lower than those obtained with ILs + 20 wt % EC, particularly for mixtures involving N1114TFSI ammonium salt, whereas for N1116TFSI and N1123TFSI, the decrease is weaker (less than 10%). This indicates that the addition of LiTFSI increases the ion association. However, the ion association increase is lower than in the absence of EC,34,17 which was attributed to the formation of “strong” ionic aggregates from Raman and NMR evidence. Thus, EC addition presumably induces specific Li+/ EC interactions, which are not inhibited in the presence of TFSI−-based ILs32 and may limit the aggregate formation. 3.4. Overhauser Effects. NMR techniques based on the measurement of Overhauser effects could probe directly the presence of pair correlations inside liquids. In these techniques, through-space dipolar couplings are probed via cross-relaxation processes between two specific nuclei.35 These methods are powerful to probe the local structure, ion solvation sphere, and also ion-pairing inside electrolytes.36 Figure 4 presents the 7Li−1H HOESY 2D spectrum for the N1116TFSI + 20% EC + LiTFSI 0.75 M measured at 313 K. Two correlation signals are observed, one between the Li+ cation and N-methyl groups of the N1116 ammonium cation (CH3N, 3.0 ppm) and the other between the Li+ cation and EC (4.5 ppm). The first one was already observed in mixtures of similar ammonium salts and LiTFSI and was assigned to unexpected privileged interactions between the alkali cation and
Figure 5. Cyclic voltammograms in the anodic and cathodic regions for N1123TFSI and the N1123TFSI + EC mixtures with different EC contents. Platinum working and counter electrodes. Scan rate = 5 mV/ s, ambient temperature.
on N1123TFSI-based electrolyte, as this IL exhibits the higher oxidation stability.17 Oxidation occurs at high potential, close to 5.8 V vs Li/Li+ even with the addition of a large amount of EC. Regarding the cathodic part (Figure 5), the reduction of N1123TFSI without solvent is observed close to 1 V vs Li/Li+. This reduction wall is associated with imide anion reduction38 and with the lack of passive layer formation.39 In the presence of only 10 wt % EC, the reduction process is observed close to 1 V vs Li/Li+, a potential similar to those obtained without solvent. From 20 wt % EC, the reduction wall is shifted in the cathodic side as far as −0.5 V vs Li/Li+. This evolution may be associated with the formation of a passive layer occurring during the reduction of EC.40
Figure 4. 7Li−1H HOESY spectrum of N1116TFSI + LiTFSI measured at 313 K. 7Li and 1H spectra are presented as 1D traces, with assignments (mixing time = 500 ms, T = 313 K). Inset: build-up magnetization for ammonium CH3N site and EC site normalized by the effective number of hydrogens on each site. 7717
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4. CONCLUSIONS The incorporation of carbonate solvent in alkyl ammonium bis(trifluoromethane sulfonyl) imide ionic liquids has been studied. With up to 30 wt % of solvent, Tg and melting temperature of ILs decrease; for a larger amount of solvent, some phase separation seems to occur. The addition of a low amount of solvent, that is, 20 wt % permits us to increase notably the conductivity values, reaching 12 mS/cm at 40 °C. The addition of LiTFSI in IL + solvent electrolytes induces a conductivity decrease, but lower than that observed in pure ILs. The incorporation of 20 wt % EC has no positive effect on the pure IL dissociation; this was not expected, taking into account the high dielectric constant of EC. Contrary to that observed in pure ILs, a good ion dissociation between 40% and 50% is obtained with the electrolyte IL + EC + LiTFSI, due to Li+/EC interaction. The correlations between Li+/EC and Li+/NCH3 have similar intensities, which evidences that the Li+ cation is solvated by EC but also that ionic clusters, [Li(TFSI)n+1]n−, remain even after EC incorporation. Furthermore, the incorporation of EC in ILs improves the electrochemical stability windows crucial for the lithium battery application. In conclusion, the EC−IL mixtures constitute an interesting alternative and safe solvents for application in lithium batteries.
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AUTHOR INFORMATION
Corresponding Author
*Phone: +33 4 76 82 65 61. E-mail:
[email protected]. Present Addresses ∥
Vietnam National University, VNU-HCM, Ho Chi Minh City, Vietnam. ⊥ Laboratoire Léon Brillouin, UMR 12, CEA-CNRS, 91191Saclay, France. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank the Region Rhône-Alpes, which awarded a grant to M.L.P. Le REFERENCES
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